PPDL Picture of the Week
October 1, 2018
Horseweed
Joe Ikley, Weed Science Program Specialist,
Department of Botany and Plant Pathology, Purdue University
Harvest
time is often one of the best times of the year to evaluate how herbicide
programs worked for season-long weed control. Weed escapes are easy to spot
from the height of a combine cab, and many of us have created new four-letter
words if we plug up our combines with still-green weeds. What is often more
difficult to determine, is what weeds are newly emerged and utilizing sunlight
that is reaching the soil surface as our crops dry down. We have already found
emerged horseweed (marestail) rosettes growing in between crop rows prior to
harvest (picture 1). As long as there is bare soil, and adequate moisture, we
can often find horseweed germinating from late August throughout November
across much of Indiana. Once our crops are out of the field, these emerged
plants will capture as much sunlight and nutrients as they can until the ground
freezes in order to successfully overwinter and become next year’s headache.
An
early harvest this year means we will have more time to apply fall burndown
herbicides for horseweed and other problematic winter annual weeds. If these
applications are applied soon after harvest, this will be a year where residual
herbicides can be of benefit to help keep fields weed-free until the ground
freezes. Applications that are made closer to ground-freeze often do not
receive as much benefit from the addition of residual herbicides, as there is
less time available for new weeds to germinate this calendar year. Many
effective options can be found in Table 1 of a newsletter article we published
in Pest and Crop newsletter in 2017 about effective fall applied burndown
herbicides (https://extension.entm.purdue.edu/pestcrop/2017/Issue21/).
In general, most options that will effectively control fall-emerged horseweed
will also control other winter annual weeds we find in our fields.